There is a need for improved means to obtain or manipulate fatty acid compositions, from biosynthetic or natural plant sources. For example, novel oil products, improved sources of synthetic triacylglycerols (triglycerides), alternative sources of commercial oils, such as tropical oils (i.e., palm kernel and coconut oils), and plant oils found in trace amounts from natural sources are desired for a variety of industrial and food uses.
To this end, the triacylglycerol (TAG) biosynthesis system in plants and bacteria has been studied. In the cytoplasmic membranes of plant seed tissues which accumulate storage triglycerides ("oil"), fatty acyl groups at the sn-2 position of the triglyceride molecules are incorporated via action of the enzyme 1-acylglycerol-3-phosphate acyltransferase (E.C. 2.3.1.51), also known as lysophosphatidic acid acyltransferase, or LPAAT.
By inspection of the LPAAT activities in isolated membranes from seed tissues, it has been shown that LPAAT specificities vary from species to species in accordance with the kinds of fatty acyl groups found in the sn-2 positions of the respective storage oils. For example, in the seeds of Cuphea species, which accumulate oils containing medium-chain fatty acids, it is possible to demonstrate an LPAAT activity which will utilize medium-chain acyl-CoA and lysophosphatidic acid (LPA) substrates. In contrast, LPAAT activity from the membranes of rapeseed embryos, in which the oil contains fatty acids of longer chain length, uses these medium-chain substrates much less readily, and predominantly uses long-chain unsaturated fatty acids. Similarly the meadowfoam plant (Limnanthes alba) accumulates an oil containing erucic acid (22:1) in all three sn positions and has a seed LPAAT activity able to use 22:1-CoA and 22:1-LPA, whereas rapeseed, which does not accumulate these fatty acids, has little or no such 22:1-utilizing LPAAT.
Similar studies with the enzymes responsible for the sn-1 and sn-3 acylations show that they are much less selective with respect to the substrate chain lengths. Thus, for a specific storage triglyceride in a given plant, the types of fatty acyl groups found in the sn2 position of the oil are determined primarily by the specificity of LPAAT with respect to its acyl-donor substrates, i.e. acyl-CoAs. In addition, the selectivity of the LPAAT towards the acyl-CoAs is also influenced by the nature of the acyl group already attached in the sn-1 position of the acceptor substrates, i.e. the 1-acylglycerol-3-phosphate or lysophosphatidic acid (LPA) molecules.
The characterization of lysophosphatidic acid acyltransferase (also known as LPAAT) is useful for the further study of plant FAS systems and for the development of novel and/or alternative oils sources. Studies of plant mechanisms may provide means to further enhance, control, modify or otherwise alter the total fatty acyl composition of triglycerides and oils. Furthermore, the elucidation of the factor(s) critical to the natural production of triglycerides in plants is desired, including the purification of such factors and the characterization of element(s) and/or co-factors which enhance the efficiency of the system. Of special interest are the nucleic acid sequences of genes encoding proteins which may be useful for applications in genetic engineering.